In-plane switching liquid crystal display comprising compensation film for angular field of view using positive biaxial retardation film

ABSTRACT

Disclosed is an in-plane switching liquid crystal display, which uses a positive biaxial retardation film while adjusting an optical axis direction and the retardation value of the positive biaxial retardation film. The in-plane switching liquid crystal display improves the contrast characteristic at a predetermined angular position as well as at a front position thereof, so a color shift according to the viewing angle in the black state is minimized.

This application is a divisional of application Ser. No. 10/991,542,filed Nov. 19, 2004 now U.S. Pat. No. 7,446,838, and claims the benefitof Korean Patent Application No. 10-2003-0083023, filed on Nov. 21,2003, each of which are hereby incorporated by reference for allpurposes as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to a liquid crystal display (LCD), andmore particularly to an in-plane switching liquid crystal display(IPS-LCD) including a compensation film, which uses a positive biaxialretardation film while adjusting an optical axis direction and aretardation value of the positive biaxial retardation film in order toimprove a viewing angle characteristic of the in-plane switching liquidcrystal display having a liquid crystal cell filled with liquid crystalof positive dielectric anisotropy (Δ∈>0) or negative dielectricanisotropy (Δ∈<0).

BACKGROUND ART

Electrodes of an IPS-LCD are aligned in such a manner that an electricfield is applied in parallel to a liquid crystal plane of the IPS-LCD.Surfaces of a liquid crystal layer adjacent to two substrates havepretilt angles in a range of 0° to 5° as disclosed in U.S. Pat. No.6,078,375. An IPS panel (liquid crystal cell) has an active matrix driveelectrode comprising a pair of electrodes aligned in the same plane. Inaddition, the active matrix drive electrode provides IPS (In-PlainSwitching), S-IPS (Super-In-Plain Switching) and FFS (Fringe FieldSwitching) modes to a liquid crystal layer formed between two glasssubstrates. According to the S-IPS mode, a two-domain liquid crystalalignment can be obtained by forming a zig-zag type electrode pattern,so an IPS color shift in a white state(Bright State) may be minimized.

The IPS-LCD is disclosed in U.S. Pat. No. 3,807,831. However, theIPS-LCD disclosed in U.S. Pat. No. 3,807,831 does not use a compensationfilm. Accordingly, the IPS-LCD represents a low contrast ratio at apredetermined inclination angle due to a relatively great amount oflight leakage.

U.S. Pat. No. 5,189,538 discloses an LCD including two kinds ofretardation films, such as a +A-plate and a positive biaxial retardationfilm, but it does not disclose information or technologies about theIPS-LCD.

U.S. Pat. No. 5,440,413 discloses a TN-LCD having two positive biaxialretardation films in order to improve the contrast characteristic andcolor characteristic of the TN-LCD at a predetermined inclination angle.

An IPS-LCD compensation film using one positive biaxial retardation filmis disclosed in U.S. Pat. No. 6,285,430. Characteristics of the IPS-LCDare as follows:

-   -   One positive biaxial retardation film is aligned between a        polarizer plate and a liquid crystal layer.    -   An in-plane retardation value of the biaxial retardation film is        about 190 nm to 390 nm.    -   An in-plane retardation value of the biaxial retardation film        increases in proportional to an absolute value of a retardation        in a direction of thickness of a polarizer plate protection        film.

A main object of using the biaxial retardation film is to improve acontrast characteristic of the IPS-LCD at an inclination angle in allazimuthal angles, especially 45°, 135°, 225° and 315°. Although thecontrast characteristic of the IPS-LCD can be improved at the aboveazimuthal angles, the IPS-LCD in a black state represents a great amountof light leakage at other azimuthal angles. For this reason, a contrastratio of the IPS-LCD is relatively reduced at other azimuthal angles.Therefore, the above IPS-LCD has a disadvantage in that the IPS-LCDrepresents a relatively low contrast ratio at specific azimuthal anglesdue to relatively great light leakage in the black state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a basic structure of an IPS-LCD.

FIG. 2 is a view illustrating an alignment of an absorption axis of apolarizer plate and an optical axis of liquid crystal in IPS-LCD panelof FIG. 1.

FIG. 3 is a view illustrating a refractive index of a retardation film.

FIGS. 4 a and 4 b are views illustrating a structure of a first IPS-LCDincluding a viewing angle compensation film according to one embodimentof the present invention.

FIGS. 5 a and 5 b are views illustrating a structure of a second IPS-LCDincluding a viewing angle compensation film according to one embodimentof the present invention.

FIGS. 6 a and 6 b are views illustrating a structure of a third IPS-LCDincluding a viewing angle compensation film according to one embodimentof the present invention.

FIGS. 7 to 10 are graphs representing simulation results for a contrastcharacteristic at inclination angles of about 0° to 80° in all azimuthalangles when a white light is used in an IPS-LCD structure including aviewing angle compensation film according to one embodiment of thepresent invention, in which FIG. 7 is a simulation result of a firstIPS-LCD structure, FIG. 8 is a simulation result of a second IPS-LCDstructure, FIG. 9 is a simulation result of a third IPS-LCD structure,and FIG. 10 is a simulation result of a third IPS-LCD structure.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an IPS-LCD capableof obtaining a high contrast characteristic at all inclination angles inall azimuthal angles as well as at a front position.

Another object of the present invention is to provide an IPS-LCDrepresenting low transmittance in a black state at all inclinationangles in all azimuthal angles.

The present invention accomplishes the above object by using a positivebiaxial retardation film while adjusting an optical axis direction andthe retardation value of the positive biaxial retardation film.

In order to accomplish the above object, there is provided an in-planeswitching liquid crystal display comprising: a first polarizer plate; asecond polarizer plate; and a liquid crystal cell, which is horizontallyaligned between two glass substrates and filled with liquid crystal ofpositive dielectric anisotropy (Δ∈>0) or negative dielectric anisotropy(Δ∈<0), an optical axis of the liquid crystal filled in the liquidcrystal cell being aligned in-plane in parallel to the first and secondpolarizer plates, wherein an absorption axis of the first polarizerplate is perpendicular to an absorption axis of the second polarizerplate, the optical axis of the liquid crystal filled in the liquidcrystal cell is parallel to the absorption axis of the first polarizerplate, a positive biaxial retardation film is aligned between the liquidcrystal cell and the polarizer plate in order to compensate for aviewing angle, and an optical axis direction and a retardation value ofthe positive biaxial retardation film are adjusted according to analignment order of positive biaxial retardation films.

In order to compensate for the viewing angle of the in-plane switchingliquid crystal display (IPS-LCD) in a black state, the present inventionis characterized by using at least one positive biaxial retardation filmin which an optical axis direction and a retardation value of thepositive biaxial retardation film are adjusted according to upper andlower polarizer plates and an alignment order of the positive biaxialretardation films.

A contrast ratio is an index representing a degree of definition for animage, and a high contrast ratio allows a high definition image. Thecontrast characteristic of the IPS-LCD is most deteriorated at aninclination angle of 70°. Thus, if the IPS-LCD represents an improvedcontrast characteristic at the inclination angle of 70°, it means thatthe contrast characteristic of the IPS-LCD is improved at all viewingangles. Accordingly, comparison of improvement for viewing anglecharacteristics of IPS-LCDs is preferably carried out with theinclination angle of 70°. When the IPS-LCD uses the only polarizerplates, a minimum contrast ratio at the inclination angle of 70° isequal to or less than 10:1. However, the IPS-LCD of the presentinvention uses the positive biaxial retardation film while adjusting theoptical axis and the retardation value thereof, so the IPS-LCD of thepresent invention may represent a minimum contrast ratio above 20:1.Preferably, the IPS-LCD of the present invention represents the minimumcontrast ratio above 20:1 at the inclination angle of 70°.

Reference will now be made in detail to the present invention.

FIG. 1 is a view illustrating a basic structure of an IPS-LCD.

The IPS-LCD includes a first polarizer plate 1, a second polarizer plate2 and a liquid crystal cell 3. An absorption axis 4 of the firstpolarizer plate 1 is aligned in perpendicular to the an absorption axis5 of the second polarizer plate 2 and the absorption axis 4 of the firstpolarizer plate 1 is parallel to an optical axis 6 of the liquid crystalcell 3. In FIG. 2, two absorption axes 4 and 5 of two polarizer platesand one optical axis 6 of one liquid crystal cell are shown.

The liquid crystal display using a compensation film according to thepresent invention includes the first polarizer plate 1, the liquidcrystal cell 3, which is horizontally aligned between two glasssubstrates and filled with liquid crystal of positive dielectricanisotropy (Δ∈>0) or negative dielectric anisotropy (Δ∈<0), and thesecond polarizer plate 2. The optical axis 6 of the liquid crystalfilled in the liquid crystal cell 3 is aligned in-plane in parallel tothe polarizer plates. The absorption axis 4 of the first polarizer plate1 is aligned in perpendicular to the absorption axis 5 of the secondpolarizer plate 2 and the absorption axis 4 of the first polarizer plate1 is parallel to the optical axis 6 of the liquid crystal filled in theliquid crystal cell 3. In addition, one of the first and secondsubstrates includes an active matrix drive electrode having a pair ofelectrodes, which is formed on a surface of the substrate adjacent to aliquid crystal layer.

A retardation value of the liquid crystal layer is defined asR_(LC)=(n_(x,LC)−n_(y,LC))×d, wherein d is a thickness of the liquidcrystal layer. Preferably, the liquid crystal layer of the IPS panelaccording to the present invention has a retardation value in a range of200 nm to 400 nm at a wavelength of 550 nm.

In order to make a white state when voltage is applied to the IPS-LCDpanel, the light linearly polarized at 90° after passing through thefirst polarizer plate must be linearly polarized into 0° after it haspassed through the liquid crystal layer. Further, in order to achievethe state of light polarized as described above, the retardation valueof the liquid crystal layer of the IPS-LCD must be a half wavelength of589 nm (a monochromatic light representing a highest brightness which aperson can feel). Therefore, in order to allow the light to produce awhite color, the retardation value of the liquid crystal layer of theIPS-LCD can be adjusted to be somewhat shorter or longer than the halfwavelength of 589 nm. Therefore, the retardation value is preferably inthe range around 295 nm corresponding to the half wavelength of 589 nm.

The LCD of the present invention may align the liquid crystal inmulti-directions, or the liquid crystal may be divided intomulti-regions by voltage applied thereto.

The LCDs can be classified into IPS (In-Plain Switching) LCDs, Super-IPS(Super-In-Plain Switching) LCDs and FFS (Fringe Field Switching) LCDsaccording to modes of the active matrix drive electrode including a pairof electrodes. In the present invention, the IPS-LCD may include theSuper-IPS LCD, the FFS LCD, and a reverse TN IPS LCD.

FIG. 3 illustrates a refractive index of a retardation film used forcompensating for a viewing angle of the IPS-LCD. Referring to FIG. 3, arefractive index in an x-axis direction is n_(x)(8), a refractive indexin a y-axis direction is n_(y)(9), and a refractive index in a z-axisdirection is n_(z)(10). The characteristic of the retardation filmdepends on the refractive index.

A biaxial retardation film represents mutually different refractiveindexes in x-axis, y-axis and z-axis directions. The biaxial retardationfilm is defined as follows:n _(x) ≠n _(y) ≠n _(z)  Equation 1

A negative biaxial retardation film is defined as follows:n _(x) ≠n _(y) >n _(z)  Equation 2

A positive biaxial retardation film is defined as follows:n _(x) ≠n _(y) <n _(z)  Equation 3

The positive biaxial retardation film satisfying Equation 3 representsmutually different refractive indexes in x-axis, y-axis and z-axisdirections, so it has an in-plane retardation value and a thicknessretardation value. The in-plane retardation value can be defined asfollows by using in-plane refractive indexes of n_(x)(8) and n_(y)(9).R _(in) =d×(n _(x) −n _(y)),  Equation 4wherein d is a thickness of a film.

The thickness retardation value can be defined as follows by usingrefractive indexes of n_(y)(9) and n_(z)(10).R _(th) =d×(n _(z) −n _(y)),  Equation 5wherein d is a thickness of a film.

The positive biaxial retardation film signifies a film having thepositive in-plane retardation value and the positive thicknessretardation value.

The wavelength dispersion characteristic of the positive biaxialretardation film includes normal wavelength dispersion, flat wavelengthdispersion, and reverse wavelength dispersion. The unlimited example ofthe positive biaxial retardation film includes an UV curable liquidcrystal film using a nematic liquid crystal and a biaxially oriented PC(polycarbonate).

According to the present invention, the direction of the optical axis ofthe retardation film is determined according to an alignment order ofthe retardation film.

According to a first embodiment of the present invention, there isprovided an in-plane switching liquid crystal display including apositive biaxial retardation film 11 aligned between an IPS panel 3 anda second polarizer plate 2, in which an optical axis 12 of the positivebiaxial retardation film 11 is perpendicular to an absorption axis 5 ofthe second polarizer plate 2, with an in-plane retardation value of thepositive biaxial retardation film 11 equal to or less than 190 nm at awavelength of 550 nm.

The optical axis of the retardation film relates to light leakagegenerated from two polarizer plates with the absorption axes thereofaligned perpendicularly to each other. That is, in order to minimize thelight leakage of the polarizer plates, the in-plane optical axis of thepositive biaxial retardation film must be aligned perpendicularly to theabsorption axis of the polarizer plate adjacent to the positive biaxialretardation film.

When the absorption axes of the polarizer plates are aligned in anangular direction of 0° and 90° respectively, if an observer checks theblack state while inclining the polarizer plates in a direction of anazimuthal angle of 45°, it can be found that the light leakage increasesas the inclination angle increases. This is because the angle betweenabsorption axes of two polarizer plates greatly deviates from aperpendicular state as the inclination angle increases. That is, inorder to minimize the light leakage, the polarized light must be rotatedby an extent of angle as deviated from a perpendicular state. The lightradiated from a backlight unit is linearly polarized after passingthrough the first polarizer plate, and a rotational angle of thelinearly polarized light must be increased according to the inclinationangle. In order to rotate the linearly polarized light, the in-planeoptical axis of the positive biaxial retardation film must be aligned inperpendicular to the absorption axis of the adjacent polarizer plate.

Meanwhile, in order to allow the linearly polarized light to rotate inmatch with the absorption axis of the second polarizer platecorresponding to an increase of the inclination angle, the in-planeretardation value of the positive biaxial retardation film is preferablyset equal to or less than 190 nm. At this time, the in-plane retardationvalue of the positive biaxial retardation film may vary according to thethickness retardation value of the positive biaxial retardation film. Inorder to properly compensate for the viewing angle, it is preferred toreduce the in-plane retardation value as the total thickness retardationvalue of the retardation film increases.

The first embodiment of the present invention is illustrated in FIGS. 4a and 4 b, and the structures of the IPS-LCDs shown in FIGS. 4 a and 4 bare substantially identical to each other, except for positions of abacklight unit and an observer.

Table 1 is a simulation result for a viewing angle characteristic at aninclination angle of 70° according to design values (in-planeretardation value and thickness retardation value) of a polarizer plateprotective film and the positive biaxial retardation film under thefirst IPS-LCD structure shown in FIG. 4 a.

TABLE 1 Minimum Internal Internal contrast protective protective ratioat film of 1^(st) Positive biaxial film of 2^(nd) inclination polarizerIPS- retardation film polarizer angle plate Panel R_(in)(nm) R_(th)(nm)Nz plate of 70° COP 290 180 144 0.2  80 um TAC 166 nm 160 173 −0.08 120um TAC 83 40 um TAC 160 88 0.45  40 um TAC 83 124 102 0.18  80 um TAC 79118 139 −0.17 120 um TAC 65 80 um TAC 160 49 0.72 COP 33 155 78 0.5  40um TAC 30 110 77 0.3  80 um TAC 30

The simulation is carried out with conditions representing the superiorviewing angle characteristic at all inclination angles in all azimuthalangles by taking the retardation values of internal protective films ofthe first and second polarizer plates 1 and 2, the in-plane retardationvalue and the thickness retardation value of the positive biaxialretardation film 11, and an Nz representing biaxiality intoconsideration.

Herein, the Nz is an index representing biaxiality of the positivebiaxial retardation film, which can be defined as follows by using therefractive indexes of the film in three axis directions.

$\begin{matrix}{N_{z} = \frac{\left( {n_{x} - n_{z}} \right)}{\left( {n_{x} - n_{y}} \right)}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

Table 1 shows the improvement in the viewing angle characteristicaccording to the design values of the polarizer plate protective filmand the positive biaxial retardation film. Referring to Table 1, sincethe IPS-LCD which does not use the viewing angle compensation film has aminimum CR (contrast ratio) of about 7:1, if the IPS-LCD represents theCR above 30:1 at an inclination angle of 70°, it means that the CR valueabove 30:1 can be obtained in all viewing angles, resulting in a muchimprovement of the viewing angle characteristic.

In addition, Table 2 shows a simulation result when practical designvalues of a retardation film are applied to the IPS-LCD structure shownin FIG. 4 b.

TABLE 2 Internal Internal Minimum protective protective contrast film of1^(st) Positive biaxial film of 2^(nd) ratio at polarizer IPS-retardation film polarizer inclination plate Panel R_(in)(nm) R_(th)(nm)Nz plate angle of 70° 40 um TAC 250 170 76 0.55 40 um 83 290 160 88 0.45TAC 83 330 155 102 0.34 83

Table 2 shows the improvement in the viewing angle characteristicaccording to the design values of the polarizer plate protective filmand the positive biaxial retardation film. If 40 um TAC (triacetatecellulose) is used as a polarizer plate protective film, the polarizerplate protective film has a negative R_(th) lower than an R_(th) of thepolarizer plate protective film of 80 um TAC, so the design value of thepositive biaxial retardation film is changed. Accordingly, it ispossible to obtain a superior viewing angle characteristic by varyingthe design values. In detail, it is possible to obtain the CR more than80:1 at an inclination angle of 70° by adjusting the design values ofthe polarizer plate protective film and the positive biaxial retardationfilm.

According to a second embodiment of the present invention, there isprovided an in-plane switching liquid crystal display including a firstpositive biaxial retardation film 11 aligned between an IPS panel 3 anda first polarizer plate 1, and a second positive biaxial retardationfilm 13 aligned between the IPS panel 3 and a second polarizer plate 2,in which an optical axis 12 of the first positive biaxial retardationfilm 11 is parallel to an absorption axis 4 of the first polarizer plate1 and an optical axis 14 of the second positive biaxial retardation film13 is perpendicular to an absorption axis 5 of the second polarizerplate 2, with the first positive biaxial retardation film 11 having anin-plane retardation value equal to or less than 190 nm at a wavelengthof 550 nm and the second positive biaxial retardation film 13 having anin-plane retardation value in a range of 150 to 350 nm at a wavelengthof 550 nm.

The viewing angle characteristic of the IPS-LCD may be lowered due to ageometrical problem of the polarizer plate depending on the viewingangle and a dependency of the retardation value of the IPS-LCD panel tothe viewing angle. The black state of the LCD is obtained by using twopolarizer plates, in which a light generated from the backlight unit andlinearly polarized by the first polarizer plate is absorbed by means ofthe absorption axis of the second polarizer plate. However, unlike avertical incident light, a slantingly incident light creates a rotatedlinearly polarized light which has been rotated after passing throughthe polarizer plate, and experiences the absorption axis of the secondpolarizer plate rotated. Therefore, the linearly polarized lightintroduced through the first polarizer plate is not perpendicular to theabsorption axis of the second polarizer plate, so a light component,which is parallel to a transmission axis, is created. As the inclinationangle becomes enlarged, the linearly polarized light greatly deviatesfrom the perpendicular state with respect to the transmission axis, solight components in parallel to the transmission axis may increase. Forthis reason, the light leakage may occur in the black state.

The light leakage under the black state of the LCD is a main factorcausing deterioration of the viewing angle characteristic of the LCD.The light leakage increases according to an increase of the inclinationangle and the increase of the light leakage lowers the CR and increasesa color shift. Thus, it is possible to improve the viewing anglecharacteristic by minimizing the light leakage under the black statedepending on the inclination angle. In order to improve the viewingangle characteristic, the light which has been linearly polarized afterpassing through the first polarizer plate must match with the absorptionaxis of the second polarizer plate. To this end, the present inventionutilizes the positive biaxial retardation film. In order to allow thelight, which has been linearly polarized, to match with the absorptionaxis of the second polarizer plate according to the inclination angle,the in-plane retardation value and the thickness retardation value arenecessary.

The absorption axis of the first polarizer plate must match with theoptical axis of the first positive biaxial retardation film, so as thata predetermined elliptically polarized light can be created through thefirst positive biaxial retardation film. Then, the ellipticallypolarized light turns into a linearly polarized light matching with theabsorption axis of the polarizer plate through the second positivebiaxial retardation film. To this end, the optical axis of the secondpositive biaxial retardation film must be aligned perpendicularly to theabsorption axis of the second positive biaxial retardation film. If thefirst positive biaxial retardation film having an in-plane retardationvalue equal to or less than 190 nm is employed, the first positivebiaxial retardation film converts the light, which has been linearlypolarized through the first polarizer plate, into an ellipticallypolarized light, which is required for generating the linearly polarizedlight which matches with the absorption axis of the second polarizerplate after the light has passed through the second positive biaxialretardation film.

The second positive biaxial retardation film converts the ellipticallypolarized light formed through the first positive biaxial retardationfilm into the linearly polarized light. In addition, if the secondpositive biaxial retardation film having the retardation value in arange of about 150 nm to 350 nm is used according to the polarizingstate of the light created by the first positive biaxial retardationfilm, it is possible to obtain the linearly polarized light whichmatches with the absorption axis of the second polarizer plate.

The second embodiment of the present invention is illustrated in FIGS. 5a and 5 b, and the structures of the IPS-LCDs shown in FIGS. 5 a and 5 bare substantially identical to each other, except for positions of abacklight unit and an observer.

Table 3 shows a simulation result when practical design values of aretardation film are applied to the second IPS-LCD structure shown inFIG. 5 a or 5 b.

TABLE 3 Internal Internal protective protective Minimum film of film ofcontrast 1^(st) Positive biaxial Positive biaxial 2^(nd) ratio atpolarizer retardation film IPS- retardation film polarizer angle ofplate R_(in)(nm) R_(th)(nm) Nz Panel R_(in)(nm) R_(th)(nm) Nz plate 70°COP 25 12.5 0.5 290 nm 285 142 0.5 COP 238 35 17 0.5 230 115 0.5 40 umTAC 160 60 30 0.5 200 100 0.5 80 um TAC 55 40 um TAC 160 88 0.45 302 1510.5 COP 214 124 102 0.18 250 125 0.5 40 um TAC 136 118 139 −0.17 220 1100.5 80 um TAC 50 80 um TAC 160 49 0.72 350 175 0.5 COP 100 155 78 0.5300 150 0.5 40 um TAC 68

The above simulation is carried out with conditions representing thesuperior viewing angle characteristic at all inclination angles in allazimuthal angles by taking the retardation values of internal protectivefilms of the first and second polarizer plates 1 and 2, the in-planeretardation value and the thickness retardation value of the first andsecond positive biaxial retardation films 11 and 13, and an Nzrepresenting biaxiality into consideration.

Table 3 shows the minimum CR values at the inclination angle of 70°according to the design values (in-plane retardation value, thicknessretardation value and internal protective film) of the first and secondpositive biaxial retardation films in the IPS-LCD. The most superiorviewing angle characteristic is represented when a non oriented COP(cyclo olefin polymer) film having a thickness retardation value of 0 isused for the polarizer plate internal protective film.

According to a third embodiment of the present invention, there isprovided an in-plane switching liquid crystal display including a firstpositive biaxial retardation film 11 aligned between an IPS panel 3 anda first polarizer plate 1, and a second positive biaxial retardationfilm 13 aligned between the IPS panel 3 and a second polarizer plate 2,in which an optical axis 12 of the first positive biaxial retardationfilm 11 is parallel to an absorption axis 4 of the first polarizer plate1 and an optical axis 14 of the second positive biaxial retardation film13 is parallel to an absorption axis 5 of the second polarizer plate 2,with the first positive biaxial retardation film 11 having an in-planeretardation value equal to or less than 150 nm at a wavelength of 550 nmand the second positive biaxial retardation film 13 having an in-planeretardation value in a range of 200 to 350 nm at a wavelength of 550 nm.

The optical axis of first positive biaxial retardation film must bealigned in parallel to the absorption axis of the first polarizer platein order to convert the light into an elliptically polarized light,which is required for generating the linearly polarized light after thelight passes through the second positive biaxial retardation film.

The elliptically polarized light created by the first positive biaxialretardation film can be converted into the linearly polarized lightthrough two methods. A first method is to align the optical axis of thesecond positive biaxial retardation film in perpendicular to theabsorption axis of the second polarizer plate and a second method is toalign the optical axis of the second positive biaxial retardation filmin parallel to the absorption axis of the second polarizer plate. Inthis case, the design value of the first method is different from thatof the second method.

The retardation value of the first positive biaxial retardation film mayvary depending on the design value of the second positive biaxialretardation film and the first positive biaxial retardation film havingthe in-plane retardation value equal to or less than 150 nm can createthe elliptically polarized light required for generating the linearlypolarized light, which is parallel to the absorption axis of the secondpolarizer plate, after the light has passed through the second positivebiaxial retardation film.

In addition, the retardation value of the second positive biaxialretardation film is determined according to the retardation value of thefirst positive biaxial retardation film and the second positive biaxialretardation film having the retardation value in a range of about 200 nmto 350 nm can create the linearly polarized light, which matches withthe absorption axis of the second polarizer plate.

The third embodiment of the present invention is illustrated in FIGS. 6a and 6 b, and the structures of the IPS-LCDs shown in FIGS. 6 a and 6 bare substantially identical to each other, except for positions of abacklight unit and an observer.

Table 4 shows a simulation result when practical design values of aretardation film are applied to the third IPS-LCD structure shown inFIG. 6 a or 6 b.

TABLE 4 Internal Internal protective protective Minimum film of film ofcontrast 1^(st) Positive biaxial Positive biaxial 2^(nd) ratio atpolarizer retardation film IPS- retardation film polarizer angle ofplate R_(in)(nm) R_(th)(nm) Nz Panel R_(in)(nm) R_(th)(nm) Nz plate 70°COP 35 17 0.5 290 nm 250 125 0.5 COP 278 44 22 0.5 310 155 0.5 40 um TAC234 75 37 0.5 334 167 0.5 80 um TAC 100 40 um TAC 100 50 0.5 241 120 0.5COP 259 120 60 0.5 282 141 0.5 40 um TAC 235 145 72 0.5 314 157 0.5 80um TAC 94 80 um TAC 123 62 0.5 180 90 0.5 COP 136 145 72 0.5 239 120 0.540 um TAC 100

The above simulation is carried out with conditions representing thesuperior viewing angle characteristic at all inclination angles in allazimuthal angles by taking the retardation values of internal protectivefilms of the first and second polarizer plates 1 and 2, the in-planeretardation value and the thickness retardation value of the positivebiaxial retardation film, and an Nz representing biaxiality intoconsideration. In order to simplify the simulation, “Nz=0.5” is adoptedin Table 4. However, it is also possible to use other values of the Nz.

Table 4 shows the minimum CR values at the inclination angle of 70°according to the design values when the absorption axis of the firstpolarizer plate is parallel to the optical axis of the first positivebiaxial retardation film and the absorption axis of the second polarizerplate is parallel to the optical axis of the second positive biaxialretardation film. The minimum CR value at the inclination angle of 70°may vary according to the design values of the first and second positivebiaxial retardation films and the polarizer plate internal protectivefilm. The most superior viewing angle characteristic is represented whena non oriented COP (cyclo olefin polymer) film, which has no in-planeretardation value, is used for the polarizer plate internal protectivefilm.

The polarizer plate may use internal and external protective filmshaving unique negative thickness retardation values or internal andexternal protective films, which do not have the thickness retardationvalues.

The unlimited example of internal protective film includes non orientedCOP (cyclo olefin polymer), 40 um TAC(triacetate cellulose), 80 umTAC(triacetate cellulose) or PNB (polynobonene).

The thickness retardation value of the polarizer plate internalprotective film is a very important factor when designing theretardation film such that the IPS-LCD represents low transmittanceunder a dark state at all inclination in all azimuthal angles.

The internal protective film of the first polarizer plate 1 preferablyhas a thickness retardation value of 0 or a negative thicknessretardation value. This is because the positive biaxial retardation filmadjacent to the first polarizer plate 1 may compensate for theretardation value generated from the inner protective film of the firstpolarizer plate 1.

In addition, the positive biaxial retardation film can be used as aninner protective film of at least one polarizer plate.

Preferably, the positive biaxial retardation films 11 and 13 are madefrom polymer materials or UV curable liquid crystal films.

The Nz

$\left( {N_{z} = \frac{\left( {n_{x} - n_{z}} \right)}{\left( {n_{x} - n_{y}} \right)}} \right)$representing the biaxiality of the biaxial retardation film in thepresent LCD may have various values.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed. However, it is noted that the preferred embodiments describedbelow are used for illustrative purpose and the present invention is notlimited thereto.

Embodiment 1

The IPS-LCD shown in FIG. 4 a includes an IPS liquid crystal cell filledwith liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°,dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1. An UVcurable liquid crystal film having an in-plane retardation valueR_(in)=180 nm and a thickness retardation value R_(th)=144 nm at awavelength of 550 nm is used for a positive biaxial retardation film 11.A COP internal protective film having a retardation value of almost 0 isused as an internal protective film of a first polarizer plate 1 and 80μm TAC having a thickness retardation value of R_(th)=−64 nm at awavelength of 550 nm is used as an internal protective film of a secondpolarizer plate 2. When a white light is used, the simulation result forthe contrast characteristic of a first IPS-LCD structure including aviewing angle compensation film at an inclination angle of about 0° to80° in all azimuthal angles is illustrated in FIG. 7 and Table 1.

Referring to FIG. 7, a center of a circle corresponds to an inclinationangle of 0, and the inclination angle increases as a radius of thecircle becomes enlarged. Numerals 20, 40, 60 and 80 marked along theradius of the circle in FIG. 7 represent the inclination angles.

In addition, numerals 0 to 360 marked along a circumference of thecircle represent the azimuthal angles. FIG. 7 shows the contrastcharacteristic in all viewing directions (inclination angles of 0° to80° and azimuthal angles of 0° to 360°) when an upper polarizer plate isaligned in a direction of an azimuthal angle of 0°, and a lowerpolarizer plate is aligned in a direction of an azimuthal angle of 90°.An IPS-LCD, which exclusively uses two polarizer plates, represents acontrast ratio equal to or less than 10:1 at an inclination angle of70°. However, the IPS-LCD of the present invention represents a contrastratio above 166:1 at an inclination angle of 70° as shown in FIG. 7 andTable 1.

Embodiment 2

The IPS-LCD shown in FIG. Sb includes an IPS liquid crystal cell filledwith liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°,dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1. An UVcurable liquid crystal film having an in-plane retardation valueR_(in)=20 nm and a thickness retardation value R_(th)=10 nm at awavelength of 550 nm is used for a first positive biaxial retardationfilm 11. In addition, an UV curable liquid crystal film having anin-plane retardation value R_(in)=288 nm and a thickness retardationvalue R_(th)=144 nm at a wavelength of 550 nm is used for a secondpositive biaxial retardation film 13. Internal protective films ofpolarizer plates 1 and 2 are made from COP. When a white light is used,the simulation result for the contrast characteristic of a secondIPS-LCD structure including a viewing angle compensation film at aninclination angle of about 0° to 80° in all azimuthal angles isillustrated in FIG. 8.

Embodiment 3

The IPS-LCD shown in FIG. 6 a includes an IPS liquid crystal cell filledwith liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°,dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1. An UVcurable liquid crystal film having an in-plane retardation valueR_(in)=87 nm and a thickness retardation value R_(th)=17.5 nm at awavelength of 550 nm is used for a first positive biaxial retardationfilm 11. In addition, an UV curable liquid crystal film having anin-plane retardation value R_(in)=241 nm and a thickness retardationvalue R_(th)=120 nm at a wavelength of 550 nm is used for a secondpositive biaxial retardation film 13. An internal protective film of afirst polarizer plate 1 is made from 40 μm TAC having a thicknessretardation value R_(th)=−32 nm, and the second positive biaxialretardation film 13 is used as an internal protective film of a secondpolarizer plate 2.

When a white light is used, the simulation result for the contrastcharacteristic of a third IPS-LCD structure including a viewing anglecompensation film at an inclination angle of about 0° to 80° in allazimuthal angles is illustrated in FIG. 9.

Embodiment 4

The IPS-LCD shown in FIG. 6 b includes an IPS liquid crystal cell filledwith liquid crystal having a cell gap of 2.9 μm, a pretilt angle of 3°,dielectric anisotropy of Δ∈=+7, and a birefringence of Δn=0.1. An UVcurable liquid crystal film having an in-plane retardation valueR_(in)=35 nm and a thickness retardation value R_(th)=17.5 nm at awavelength of 550 nm is used for a first positive biaxial retardationfilm 11. In addition, an UV curable liquid crystal film having anin-plane retardation value R_(in)=240 nm and a thickness retardationvalue R_(th)=120 nm at a wavelength of 550 nm is used for a secondpositive biaxial retardation film 13. Internal protective films of firstand second polarizer plates 1 and 2 are made from COP.

When a white light is used, the simulation result for the contrastcharacteristic of a third IPS-LCD structure including a viewing anglecompensation film at an inclination angle of about 0° to 80° in allazimuthal angles is illustrated in FIG. 10.

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, the in-plane switching liquid crystaldisplay according to the present invention uses the positive biaxialretardation film while adjusting the optical axis direction and theretardation value of the positive biaxial retardation film, so thein-plane switching liquid crystal display can improve the contrastcharacteristic at a predetermined angular position as well as at a frontposition thereof, so that a color shift according to the viewing anglein the black state can be minimized.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings, but, on the contrary, it isintended to cover various modifications and variations within the spiritand scope of the appended claims.

1. An in-plane switching liquid crystal display comprising: a firstpolarizer plate; a second polarizer plate; and a liquid crystal cell,which is horizontally aligned and filled with liquid crystal of positivedielectric anisotropy (Δ∈>0) or negative dielectric anisotropy (Δ∈<0),an optical axis of the liquid crystal filled in the liquid crystal cellbeing aligned in-plane in parallel to the first and second polarizerplates, wherein an absorption axis of the first polarizer plate isperpendicular to an absorption axis of the second polarizer plate, theoptical axis of the liquid crystal filled in the liquid crystal cell isparallel to the absorption axis of the first polarizer plate, a firstpositive biaxial retardation film defined by a following Equation isaligned between the liquid crystal cell and the first polarizer plate, asecond positive biaxial retardation film defined by the followingEquation is aligned between the liquid crystal cell and the secondpolarizer plate, an optical axis of the first positive biaxialretardation film is parallel to the absorption axis of the firstpolarizer plate, an optical axis of the second positive biaxialretardation film is perpendicular to the absorption axis of the secondpolarizer plate an in-plane retardation value of the first positivebiaxial retardation film is equal to or less than 190 nm at a wavelengthof 550 nm, and an in-plane retardation value of the second positivebiaxial retardation film is in a range of 150 nm to 350 nm at awavelength of 550 nm, Equation n_(x)#n_(y)<n_(z), wherein n_(x) andn_(y) are in-plane refractive indexes, n_(z), is a thickness refractiveindex, and the positive biaxial retardation film has a positive in-planeretardation value (R_(inh)=d x (n.−n_(y))) and a positive thicknessretardation value (R_(th)=d x (n_(z)−n_(y))) in which d is a thicknessof a film.
 2. The in-plane switching liquid crystal display according toclaim 1, wherein a retardation value of the liquid crystal cell is in arange of 200 nm to 400 nm at a wavelength of 550 nm.
 3. The in-planeswitching liquid crystal display according to claim 1, wherein thepositive biaxial retardation film is used as a protective film for atleast one polarizer plate.
 4. The in-plane switching liquid crystaldisplay according to claim 1, wherein an internal protective film of thefirst polarizer plate has a thickness retardation value of 0 or anegative thickness retardation value.
 5. An in-plane switching liquidcrystal display comprising: a first polarizer plate; a second polarizerplate; and a liquid crystal cell, which is horizontally aligned andfilled with liquid crystal of positive dielectric anisotropy (Δ∈>0) ornegative dielectric anisotropy(Δ∈<0), an optical axis of the liquidcrystal filled in the liquid crystal cell being aligned in-plane inparallel to the first and second polarizer plates, wherein an absorptionaxis of the first polarizer plate is perpendicular to an absorption axisof the second polarizer plate, the optical axis of the liquid crystalfilled in the liquid crystal cell is parallel to the absorption axis ofthe first polarizer plate, a first positive biaxial retardation filmdefined by a following Equation is aligned between the liquid crystalcell and the first polarizer plate, a second positive biaxialretardation film defined by the following Equation is aligned betweenthe liquid crystal cell and the second polarizer plate, an optical axisof the first positive biaxial retardation film is parallel to theabsorption axis of the first polarizer plate, an optical axis of thesecond positive biaxial retardation film is parallel to the absorptionaxis of the second polarizer plate, an in-plane retardation value of thefirst positive biaxial retardation film is equal to or less than 150 nmat a wavelength of 550 nm, and an in-plane retardation value of thesecond positive biaxial retardation film is in a range of 200 nm to 350nm at a wavelength of 550 nm, Equation n_(x)≠n_(y)<n_(z), wherein n_(x)and n_(y) are in-plane refractive indexes, n_(z) is a thicknessrefractive index, and the positive biaxial retardation film has apositive in-plane retardation value (R_(in)=d x (n_(x)−n_(y))) and apositive thickness retardation value (R_(th=d x (n) _(z)−n_(y))) inwhich d is a thickness of a film.
 6. The in-plane switching liquidcrystal display according to claim 5, wherein a retardation value of theliquid crystal cell is in a range of 200 nm to 400 nm at a wavelength of550 nm.
 7. The in-plane switching liquid crystal display according toclaim 5, wherein the positive biaxial retardation film is used as aprotective film for at least one polarizer plate.
 8. The in-planeswitching liquid crystal display according to claim 5, wherein aninternal protective film of the first polarizer plate has a thicknessretardation value of 0 or a negative thickness retardation value.